This invention relates to micromachines and an improved rotor for micromachinery. The term micromachine is used to embrace many types of very small turbines or compressors. These machines can be as small as 12 mm in diameter with rotors of 4 mm in diameter.
Micromachines such as micro-gas turbines, combustion power generators, pumps and compressors are described in U.S. Pat. No. 5,932,940 (the M.I.T. patent), the disclosure of which is incorporated herein by reference. All of these machines contain a rotor comprising a disc or discs defining either a centrifugal compressor/pump or a radial inflow turbine. The material of construction is characterised by a strength to density ratio enabling a rotor speed of at least 500,000 rotations per minute. The machines are constructed using microfabrication techniques including vapour deposition and bulk wafer etching, the material of construction being common to all the structural elements.
The compressor and the turbine rotors of the devices described in the M.I.T. patent utilise a plurality of radial flow vanes. It is considered that this arrangement of blades is not desirable in micromachines for the following reasons:
(a) because the nature of construction involves planar fabrication techniques, fillets on corners are difficult to achieve and, in the absence of adequate fillets, high stress concentration at the blade root attachment decreases the fracture strength of these microelements;
(b) the placement of blades around the periphery of the discs increases the mass of the structure at the place where centrifugal stresses have the greatest effect;
(c) the plurality of blades tends to set up undesirable turbulence and pulsations in the working fluids, and the cyclic nature of the reaction between fluids and blades results in cyclic stress fluctuations (fatigue stresses) that limit the durability (fatigue life) of the rotor assembly;
(d) the maximum rotor speed is limited in part by the allowable mechanical and thermal stresses that may be imposed on the rotor structure by the plurality of radial flow vanes;
(e) the degree of rotor balance obtainable is affected by the requirement for a plurality of radial flow vanes; and
(f) the rotor disc employs blades only on one side and is subject to a bending moment, caused by centrifugal blade loading.
It is these problems that have brought about the present invention to use a bladeless or vaneless rotor in micromachines.
The use of bladeless rotors has been suggested in the context of xe2x80x9clarge scalexe2x80x9d turbines. Thus, a method for driving turbines by means of viscous drag was taught by Tesla in U.S. Pat. No. 1,061,206 and for fluid propulsion in U.S. Pat. No. 1,061,142. In both disclosures the rotor comprises a stack of flat circular discs with openings in the central portions, with the discs being set slightly apart. In the turbine embodiment the rotor is set in motion by the adhesive and viscous action of the working fluid, which enters the system tangentially at the periphery and leaves it at the center. In the fluid propulsion embodiment, fluid enters the system at the center of the rotating discs and is transferred by means of viscous drag to the periphery where it is discharged tangentially.
For fluid propulsion applications such as pumps and compressors, the fluid is forced into vortex circulation around a central point where a pressure gradient is created. This pressure gradient is such that an increasing radial distance from the center of rotation leads to an increase in pressure, with the density of the fluid and the speed of rotation determining the rate of pressure rise. If an outwardly radial flow is superimposed on the vortex circulation an increasing pressure is imposed on the fluid as it flows outwardly.
To preserve the vortex circulation, an external force must act upon the fluid, and this force must accelerate the fluid in the tangential direction as the fluid moves outwardly in order to maintain its angular velocity. This function is simply a transfer of momentum from the impeller to the fluid, and with a centrifugal compressor it may be achieved in one of two ways. A first method is to confine the fluid within a fixed boundary channel and then accelerate the channel. In an impeller of the type utilized in prior art microturbomachinery, the vanes and rotor walls form such a channel, and acceleration occurs as the fluid moves outwardly towards regions of higher impeller velocity. A second method of momentum transfer is by viscous drag and this is the principle underlying the Tesla arrangement described in the two US patents referred to above. Viscous drag always acts to reduce the velocity difference, so that in the case of a compressor where the channel walls are moving relative and parallel to the fluid, the fluid will accelerate in the direction of the channel motion. Conversely, where the fluid is moving relative and parallel to the channel walls, the channel walls will accelerate in the direction of the fluid motion.
Known bladeless or vaneless rotors have had limited success in large scale turbines. The relatively large number of parts required for their construction and the distortion of the discs that occur under high thermal and mechanical stress conditions have restricted their adoption.
It is these issues that have brought about the present invention.
According to one aspect of the present invention there is provided a micromachine including at least one bladeless rotor, said rotor being adapted to impart energy to or derive energy from a fluid.
For the micromachine, the rotor of the invention may have a disc of diameter no greater than 20 mm.
Preferably the rotor includes a shaft centrally supporting at least two closely spaced planar discs, the discs having opposed surfaces defining a fluid passageway. At least one of the discs may have one or more apertures to allow fluid to pass into or out of the fluid passageway. The apertures preferably are close to a central region of the disc. There may be two or more apertured discs, with the apertures of each disc being aligned with those of the other disc. Preferably the discs are separated by spacers.
The rotor of the invention may have a backing disc supporting a plurality of annular discs in a closely spaced coaxial array. In that arrangement, each annular disc may be mounted to the backing disc or an adjacent disc by an array of spacers. The backing disc preferably is mounted coaxially on a shaft.
The micromachine, including its rotor, preferably has a vaned stator positioned around the periphery of the bladeless rotor.
The micromachine preferably is made of material capable of operating at temperature greater than 1000xc2x0 C. The rotor most preferably is made of a material having a tensile strength to allow the rotor to run at speeds greater than 500,000 rpm at elevated temperatures associated with combustion. The rotor may be made of a single crystal material. The rotor may, for example, be formed at least in part from a material selected from silicon, silicon carbide, silicon coated with silicon carbide, and silicon coated with silicon nitride.
The rotor preferably is formed by a microfabrication technique, such as photolithography or vapour deposition.
According to a further aspect of the present invention there is provided a rotor for a micromachine, wherein the rotor includes at least a pair of closely spaced co-axially aligned discs defining opposed planar surfaces, at least one disc having at least one aperture whereby a fluid passageway is defined between the aperture, the planar surfaces and the periphery of the rotor, and wherein the rotor is bladeless and is formed of a single crystal material.
In accordance with a still further aspect of the present invention there is provided a rotor for a micromachine, wherein the rotor includes at least a pair of closely spaced co-axially aligned discs defining opposed planar surfaces, at least one disc having at least one aperture whereby a fluid passageway is defined between the aperture, the planar surfaces and the periphery of the rotor, and wherein the rotor is bladeless and manufactured of a material having a tensile strength to allow the rotor to run at speeds greater than 500,000 rpm at elevated temperatures associated with combustion.
In accordance with a still further aspect of the present invention there is provided a rotor, wherein the rotor includes a backing disc and at least one coaxially spaced annular disc supported on the backing disc by a central hub defining at least one aperture, wherein the rotor is bladeless and the annular disc defines an unimpeded fluid passage between the aperture and the periphery of the disc.
The rotor of the invention most preferably is of unitary construction. The rotor preferably is formed by a microfabrication technique, such as photolithography or vapour deposition.